A program has been initiated to determine the electronic charge-density susceptibilities, or nonlocal polarizabilities, of molecular systems. These charge-density susceptibilities are the fundamental quantities in terms of which the dispersion interaction, including many-body effects, can be concisely formulated. As a first step in their evaluation, we have determined the susceptibility of the He atom using hydrogenic Green functions within perturbation and variation schemes. We propose to calculate the susceptibility of the H2 molecule by a similar method, using the H2 ion Green function as the unperturbed Green function. The treatment will be generalized to other two-center systems and, by assuming bond additivity, to large molecules with localized electrons. The effect of overlap will also be investigated by calculating the susceptibility of two slightly overlapping H2 molecules, using accurate single molecular wave-functions. A parallel study will be conducted to approximate susceptibilities from experimental X-ray form factors and scattered intensities, or from known one- and two-particle density matrices. The susceptibilities will be used first to calculate the forces between two rigid molecules, and then generalized to include polar groups and molecular (i.e. translations, rotational, etc.) fluctuations. Finally the theory will be extended to include solvent interactions and applied to the interaction between two large nonrigid molecules embedded in a fluid medium.